Orthogonal Frequency-Division Multiplexing (OFDM), also referred to as “multicarrier modulation” (MCM) or “discrete multi-tone modulation” (DMTM), splits up and encodes high-speed incoming serial data, modulating it over a plurality of different carrier frequencies (called “subcarriers”) within a communication channel to transmit the data from one user to another. The serial information is broken up into a plurality of sub-signals that are transmitted simultaneously over the subcarriers in parallel.
By spacing the subcarrier frequencies at intervals of the frequency of the symbols to transmit, the peak power of each modulated subcarrier lines up exactly with zero power components of the other modulated subcarriers, thereby providing orthogonality (independence and separability) of the individual subcarriers. This allows a good spectral efficiency (close to optimal) and minimal inter-channel interference (ICI), i.e. interferences between the subcarriers.
For these reasons, OFDM is used in many applications. Many digital transmission systems have adopted OFDM as the modulation technique such as digital broadcasting terrestrial TV (DVB-T), digital audio broadcasting (DAB), terrestrial integrated services digital broadcasting (ISDB-T), digital subscriber line (xDSL), WLAN systems, e.g. based on the IEEE 802.11a/g standards, cable TV systems, etc.
However, the advantage of the OFDM can be useful only when the orthogonality is maintained. In case the orthogonality is not sufficiently warranted by any means, the performances of the OFDM system may be degraded due to inter-symbol interference (ISI) and inter-carrier interference (ICI).
OFDM can also be used for OFDM Access system which is a multi-user version of the OFDM. Multiple access is achieved in OFDMA by assigning subset of subcarriers to individual users. This allows simultaneous low data rate transmission from several users.
OFDMA based cellular systems and OFDM WLAN networks suffer from interference, mainly inter-cell interference at the cell boundary, especially when all frequency channels are fully reused.
In other words, some means of mitigating the inter-cell interference (ICI) is required to support a full frequency-reuse operation. According to standards and literature, the inter-cell interference mitigation techniques include inter-cell interference coordination technique, inter-cell interference randomization and inter-cell interference cancellation technique which is better known as the interference rejection combining (IRC) technique, which takes advantage of the interference statistics (correlation property of co-channel interference) received at multiple antennas.
The inter-cell interference coordination technique or the inter-cell interference randomization technique can contribute in decreasing the inter-cell interference (ICI) but can never cancel it totally. Furthermore, it cannot decrease other kinds of interferences. Inter-cell interference cancellation is however the final desired solution.
Existing cancellation techniques are only applicable in a multi-receiving antennas OFDM receivers and they are very complex in terms of implementation especially if the interference cancellation must be accomplished at the moment (or before) of the starting of channel estimation. Knowing that interference is very harmful to channel estimation, from where the interest of canceling the interference before starting the channel estimation.
FIG. 1 shows a clarification of the meaning of interference in the situation of inter-cell interference. Other types of interferences may also occur.
Noise and interferences are added to the transmitted signal during its transmission over the air.
In the FIG. 1, M receivers are depicted, each having an antenna Rx1, . . . RxM. Hi[k] and Zi[k] represents respectively the channel gain and additive noise/interference for the kth subcarrier (k represents the discrete frequency domain) of the ith receiver.
For the transmitted signal X[k], the received signal by the ith receiver is expressed as:Yi[k]=Hi[k]X[k]+Zi[k]                K=[0, N−1], where N is the number of subcarriers.        
In the case of WLAN 802.11a, N=64.
According to this equation, the received OFDM symbol subcarrier Yi[k] is impacted by the interference Zi[k]. In general, during the preamble, the received OFDM symbol Yi[k] is used by the channel estimation block of the receiver. If Yi[k] is corrupted only by noise (random signal with known distribution), the channel estimator can handle this situation because it is an estimator. However, if Yi[k] is corrupted by interference as well, the interference is not known to the estimator and thus estimation error will occur. When the channel estimation is faulty due to the interference, the channel equalization during data detection will be faulty as well.
FIGS. 2a, 2b, 2c, 2d show an example of WLAN IEEE 802.11a with 16QAM modulation scheme.
The FIGS. 2a and 2c represent the situation before channel equalization. The FIGS. 2b and 2d represent the situation after channel equalization.
The FIGS. 2a and 2b represent a situation where only noise occurs.
The FIGS. 2c and 2d represent a situation where some interference occurs.
In the FIG. 2c, we can see the channel equalization is successful, whereas in the FIG. 2d, the channel equalization leads to an insufficient result. The interference in the received signal led to a number of false symbol decisions. Increasing the power of the multicarrier interference signal increases the spread of the constellation and leads to a further degradation in the symbol error probability.
FIG. 3 shows interference scenarios. Scenarios 1 and 4 are narrow-band scenarios. Scenario 3 is a wide-band scenario. Scenario 2 is in between.
The invention proposes a new method to cancel or dramatically reduce the interferences that the OFDM receiver receives during channel estimation. This method is very simple and efficient.